1. Field of the Invention
The present invention relates generally to methods of injection molding and housings formed therefrom. More particularly, the present invention relates to a housing of an electronic device formed by double shot injection molding.
2. Description of the Related Art
Injection molding is one of the most popular processes for manufacturing plastic products. The injection molding process generally includes (1) injecting molten plastic material into a closed mold, (2) allowing the plastic to cool down and solidify, and (3) ejecting the finished product from the mold. This process may for example be used to form enclosures or housings for various electronic products.
Referring to FIG. 1, in order to mold a single opening enclosure for electronic devices using standard injection molding processes, the mold 10 typically consists of two parts, a first mold half 12 that includes a cavity 14, and a second mold half 16 that includes a core 18. When the two mold halves 12 and 16 close, the core 18 is placed inside the cavity 14, and plastic is injected through a gate into the open space 22 found between the cavity 14 and the core 18 thereby forming a box like part with an open end. The part is typically formed vertically along its longitudinal axis 24. After allowing the part to cool, the two mold halves 12 and 16 open and the solidified part is ejected out of the mold, and more particularly the cavity 14 of the first mold half 12.
In order to allow proper removal of the core 18 when the mold halves 12 and 16 are opened, the core 18 typically includes a draft angle 28 on each of its sides (e.g., four sides). The draft angle 28 is the amount of taper required to allow the proper removal of the core 18 from the molded part along the axis 24. That is, the draft angle 28 allows the core 18 to slide out of the molded part when the molds are opened. The larger the draft angle 28, the easier it is to get the core 18 out of the part. If there is no draft angle 28 the core 18 may be difficult to remove from the molded part (the part shrinks around the core). Although the draft angle depends on the part design, in most cases a 2 degree draft angle per side is used. However, the minimum requirement is typically 1 degree and in some cases the draft angle may be as small as ½ degree. However, in elongated parts that extend longitudinally as shown, the draft angle 28 tends to be on the high side as for example at least one degree and more likely 2 degrees. If a large draft angle is not used, the sticktion force between the part and the core 18 is difficult to overcome. And even if the sticktion force is overcome, stresses may be induced in the molded part and/or the part may be damaged during ejection. A large sticktion force is typically caused by the large surface area between the part and the core in the direction of the release (e.g., along the longitudinal axis 24).
Unfortunately, because of the draft angle 28, the inside walls of the molded part are also tapered and as a result the thickness of the molded part is non-uniform. The thickness varies longitudinally from thin at the open end 30 to thick at the closed end 32. While this may not be too problematic in low depth parts, it can be especially problematic in elongated parts, as the thickness can become very large at the closed end. In enclosures for electronic devices, especially small handheld electronic devices, either the outer dimensions of the part have to grow to provide the internal space necessary for the internal electronics or the device is left with less room for these components. That is, the draft either reduces the amount usable space inside the enclosure or drives the outside of the enclosure larger to create the same space for the electronic components inside. Neither of these results is desirable in small handheld devices where the outer dimensions are highly controlled and the internal space is at a premium. Furthermore, the thick wall sections may yield cosmetic issues such as sinks, cooling/flatness issues, etc. and require additional plastic material that is not needed thereby driving up the cost of the product.
In some cases, it may be desirable to place internal features such as protrusions, recesses, undercuts, on the inside surfaces of the molded part. In cases such as these, the core may include a mechanical action. The mechanical action forces the part off of the core and at the same time causes the part to be lifted away from the internal feature thereby allowing the core to be released from the molded part. If the core was not lifted away, the part would get stuck on the core as its removed along the longitudinal axis. By way of example, the core may include a lifter that forms the internal feature on the inside surface and then moves away from the internal feature in order to provide enough clearance during removal.
Unfortunately, mechanical actions require large cores so they are not possible with smaller parts, especially small parts that are thin and long (as shown). With parts such as these, there is simply not enough room inside the part for mechanical actions such as those created by lifters. This is especially true at the closed end of the part. Even if a lifter was somehow placed on the core, it probably wouldn't make too many cycles before it failed. Because of the small size, the lifter would overheat and self destruct.
Injection molding is not limited to forming parts as described above. In some cases, injection molding may be used to put decorative features on the outer surfaces of a part. This may be accomplished using a technique called double shot injection molding. In double shot injection molding, the molding process utilizes two injections. One of the injections is used to form the part (as described above for example), and the second injection is used to create an outer layer around some or all of the part (or vice versa). By way of example, double shot injection molding may be used to place a soft layer on top of a hard layer, a transparent layer on top of an opaque layer, or create multicolored layers. In the case of an enclosure as discussed above, double shot injection molding typically only serves to enhance the look and feel of the enclosure. It does not help form the enclosure itself, as for example each of the various walls
Thus, there is a need for improved approaches for molding thin elongated enclosures, especially those for small handheld electronic devices.